Thermally conductive composites have become interesting candidate materials for numerous applications, particularly for thermal management of electronics. Modern microelectronic chips owing to their extremely high power densities require very high conductivities from materials used as gap fillers, heat spreaders, thermal pads and pastes, and thermal interface materials (TIMs), which are vital components of the entire thermal management package and are critical to ensuring good thermal contact between the chips and the metal heat sinks. The aforementioned materials are typically polymers filled with metal (aluminum and silver) or ceramic (boron nitride, aluminum nitride, silicon carbide, etc.) particles to boost their thermal conductivities. Such conventional materials are, however, reaching the limits of performance and the thermal management engineers are seeking higher thermal conductivity polymeric materials.
Another desirable characteristic of polymeric materials used in the thermal management of electronics, particularly consumer electronic devices such as laptops, cellphones, etc, is their light weight. Metal or ceramic fillers are typically high density materials and add undesirable weight to the composites. Consequently, it is worth considering graphite, which owing to its relatively low density (2100 kg/m3) and very high thermal conductivity (up to ˜2000 W/m-K), serves as an excellent alternative to conventional metal and ceramic fillers. The nanoscale manifestations of graphite are well known for their extremely high thermal conductivities and have great potential to be used as fillers in materials for thermal management applications. However, a glance at the literature tells a different story, and after years of extensive research it can now be concluded that the nanofillers, when used alone, owing to their size, lead to numerous interfaces in the composites and therefore a large aggregate thermal interface resistance in the composites. As a result, the thermal conductivities of polymeric nanocomposites have remained quite low, or about 10 W/m-K in some embodiments.
Various researchers have investigated graphite, graphene and other compounds exhibiting sought after properties such as electrical insulation or conduction and thermal conduction, for example:
U.S. Patent Application Publication 2012/0302668 relates to a semiconductor sealing material composition, including: 9.0-13 wt % of an epoxy resin; 6-7 wt % of a hardener; 0.2-0.3 wt % of a curing catalyst; 0.60-0.68 wt % of at least one additive selected from the group consisting of a coupling agent, a release agent and a coloring agent; and 79-84 wt % of a filler, wherein the filler is nano-graphene plate powder. The semiconductor sealing material composition reportedly has excellent crack resistance at a high temperature of 270° C. or more and has high thermal conductivity and excellent flame retardancy.
U.S. Patent Application Publication 2012/0296012 relates to formulations containing a mixture of an epoxy resin and an ionic liquid or an adduct of an epoxy resin and an ionic liquid which may initiate curing of the epoxy resin, the mixture having nano-materials dispersed or dissolved therein. These formulations reportedly can be used for the preparation of nanocomposites. Methods of preparing nanocomposites by curing a dispersion of nano-materials in a mixture of an epoxy resin and an ionic liquid or an adduct of an epoxy resin and an ionic liquid which may initiate curing of the epoxy resin are disclosed. The nanocomposites comprise a cured product formed by curing an epoxy resin with an ionic liquid or an adduct of an epoxy resin and an ionic liquid having nano-materials dispersed or dissolved therein. Embodiments reportedly permit manufacture of nanocomposites having relatively high fracture toughness, relatively high loadings of nano-materials and the ability to tailor the properties of the nano-composites.
U.S. Patent Application Publication 2012/0229981 relates to an electrically insulating and thermally conductive composition including 5-80 parts by weight of a resin, 20-95 parts by weight of an electrically insulating and thermally conductive powder, and 0.0001-2 parts by weight of a graphene. Another embodiment provides an electronic device including the electrically insulating and thermally conductive composition.
U.S. Patent Application Publication 2012/0142832 relates to compositions comprising at least one polymer binder, graphene sheets, and graphite, wherein the ratio by weight of graphite to graphene sheets is from about 40:60 to about 98:2.
U.S. Patent Application Publication 2010/0140792 relates to a procedure for bulk scale preparation of high aspect ratio, 2-dimensional nano platelets comprised of a few graphene layers. Use of these nano platelets in applications such as thermal interface materials, advanced composites, and thin film coatings reportedly provide material systems with superior mechanical, electrical, optical, thermal, and antifriction characteristics.
U.S. Patent Application Publication 2010/0000441 relates to a nano graphene platelet-based conductive ink comprising: (a) nano graphene platelets (preferably un-oxidized or pristine graphene), and (b) a liquid medium in which the nano graphene platelets are dispersed, wherein the nano graphene platelets occupy a proportion of at least 0.001% by volume based on the total ink volume. The ink can also contain a binder or matrix material and/or a surfactant. The ink may further comprise other fillers, such as carbon nanotubes, carbon nano-fibers, metal nano particles, carbon black, conductive organic species, etc. The graphene platelets preferably have an average thickness no greater than 10 nm and more preferably no greater than 1 nm. These inks can be printed to reportedly form a range of electrically or thermally conductive components.
Chinese Publication CN102102001 relates to a high thermal conductivity graphene-based epoxy resin adhesive and a preparation method thereof. The epoxy resin adhesive comprises the following raw materials by weight percent: 40%-60% of graphene-based epoxy resin, 2%-10% of reaction diluent, 3%-7% of curing agent, 1%-3% of promoting agent, 0.5%-1.5% of coupling agent and high performance thermal conductive filler. The high thermal conductivity graphene-based epoxy resin adhesive is based on the high thermal conductivity of graphene; under the preset technological conditions, graphene is reportedly evenly dispersed in epoxy resin matrix; by increasing the high thermal conductivity of the resin matrix and adding a small amount of high performance thermal conductive filler, the prepared adhesive reportedly has high thermal conductivity and low density and is especially suitable for the thermal conductive packaging of high-end fine electrical and electronic components.
Chinese Publication CN102127324 relates to a preparation method of a modified graphene oxide, comprising the following steps of: (a) reacting a phosphorus oxychloride compound, cyanuric chloride or diisocyanate with glycidol to obtain an intermediate product; and (b) dispersing graphite oxide in an organic solvent, dropwise adding to the intermediate product obtained through reaction, and reacting to obtain the modified graphene oxide. The publication also provides a preparation method of a composite material containing the modified graphene oxide, comprising the following steps of: dispersing the modified graphene oxide obtained through the preparation method in the technical scheme in an organic solvent, mixing the modified graphene oxide containing epoxide groups with epoxy resin oligomers and a polyamide curing agent, and curing to obtain the composite material containing the modified graphene oxide. In the composite material provided, the modified graphene oxide reportedly reacts with a resin substrate, is more uniform to disperse in the obtained composite material, and has better flame-retarding performance.
Chinese Publication CN102153835 relates to a modified graphene/epoxy resin composite material and a preparation method thereof. The modified graphene/epoxy resin composite material has a two-phase structure, wherein an epoxy resin substrate serves as a main body, and black modified graphene serves as a wild phase. A preparation process comprises the following steps of: preparing graphite oxide; preparing graphene oxide; and preparing a modified graphene/epoxy resin composite material. The modified graphene/epoxy resin composite material reportedly has higher toughness compared with epoxy resin and a graphene/epoxy resin composite material, the interface bonding performance between graphene and epoxy resin is greatly enhanced, and the utilization of the performance of graphene is facilitated. By adopting the preparation method of the modified graphene/epoxy resin composite material, the reaction temperature for the preparation of graphite oxide serving as an intermediate product is raised, thus the reaction speed is increased.
Chinese Publication CN102443247 relates to a preparation method of graphene oxide grafted POSS (polyhedral oligomeric silsesquioxane) modified epoxy resin. The preparation method comprises the following steps of: taking crystalline flake graphite as a raw material, preparing graphite oxide through adopting a Hummers oxidation method, further adding the graphite oxide into distilled water, forming a uniformly-dispersed graphene oxide mixed solution under an ultrasonic environment, further adding POSS and a strong catalyst, performing full reaction, then filtering, washing and drying to get black powder; and further uniformly dispersing graphene oxide/POSS in an epoxy resin matrix, cross-linking and bonding under the action of a curing agent to get a composite material, molding by casting, cooling and demolding. According to the preparation method disclosed by the invention, the POSS is successfully grafted on a graphene oxide sheet layer, and advantages of the graphene oxide structure and the POSS structure are reportedly complemented, so that the preparation method has the advantages of low cost and easy obtaining of the raw material, easy operation, simple process, good reproducibility and obvious toughening modification effect against the epoxy resin.
However, the art still needs high thermally conductive composites that are relatively low density and include a thermally conductive filler mixture that exhibit relatively low thermal resistance at filler interfaces.